1. Field of the invention
This invention relates to a biologic magnetometer for measuring electric currents biologically generated in a living organ so as to estimate position of active area of the living organ
2. Description of the Related Arts
In recent years the development of magnetometers employing superconductive quantum interface device, referred to hereinafter as a SQUID, has been allowing its wider applications in medical diagnosis apparatus.
In analyzing defects or mechanisms of human internal organs from the measured magnetic field intensities, the location of electric current source, which induces the magnetic field, must be necessarily determined. For this analysis an inverse problem must be solved. That is, the current source must be determined by being calculated from the measured magnetic field intensities. However, this calculation is extremely difficult because the matrix to present the relation between the currents and the magnetic field intensities is singular.
A first prior art procedure to estimate the current source location is hereinafter described. In estimating a current source of heart an infinite, homogeneous electrical conductor is hypothetically provided in the place of heart. For estimating a brain, a homogeneous sphere or cospherical multi-layer conductors are hypothetically provided in place of the brain. Next, a current source, which may be sometimes called a current dipole, is hypothetically provided on the hypothetically provided conductor. Next, magnetic field intensity generated by the current source is calculated according to Biot-Savart's law for each detection point where a plurality of pickup coils are respectively placed. Next, these calculated magnetic field intensities are compared with magnetic field intensities really measured with the pickup coils. Location and current amount of the hypothetical current source which provides the minimum difference, i.e. the least squares between the calculated magnetic field intensities and measured magnetic field intensities are searched; then it is determined as the current source. A first problem in this least squares method is in that in determining the current source a plurality of the hypothetical current source locations and a plurality of hypothetical current values thereof must be tried, consequently a considerably long time must be consumed because this try does not always result in converging the difference. There is also a second problem in that when the current sources are on plural locations the try (calculation) may lead to a similar but wrong solution called a local minimum, and of course it consumes a much longer time. Thus, it is practically impossible to obtain a unique solution or real time solution.
As a second prior art to avoid these problems, the long time consumption and the wrong solution, there is a method utilizing a singular value deposition, referred to hereinafter as SVD method. In the SVD method there are provided a plurality of pickup coils and a plurality of locations three-dimensionally placed current sources hypothetically provided, for example, on three-dimensionally configurated grids; and a simultaneous linear equation is determined according to Biot-Savart's law to present the relation between the currents and the magnetic field intensities at the pickup coils. After the equation based on the distances between the current sources and the coil locations is determined, the equation is fixed so that only the current values are tried to provide the least squares of the differences between the measured magnetic field intensities and the calculated magnetic field intensities, as well as provide the least total of the squares of the current values. In this method the current source distribution can be more quickly obtained than in the first prior art where the hypothetical current source locations are changed one by one, because the relation between the current values and the magnetic field intensities can be quickly given by using the already calculated inverse matrix of the simultaneous linear equations. This method was disclosed by Brian Jeffs, et al. on IEEE Transaction on Biomedical Engineering, vol. BME-34, No.9. September 1987.
Problem of the second prior art is in that in order to achieve a good resolution of the current source locations, the number m of the pickup coils is required to be equal to or larger than the number n of the three-dimensionally preset locations of the current sources. Accordingly, if the preset current source locations are on a several millimeter pitch on a 10 cm diameter heart, almost ten thousand of pickup coils are required together with their respective SQUID magnetometers. It is practically impossible to provide such a great number of pickup coils, as well as to provide an apparatus to display the heart movement in real time from the data measured by such a great number of the pickup coils.